Edexcel A-Level Chemistry AS Paper 2, November 2020: Question 3
6 marks · Medium difficulty · Multiple Choice
Assess reaction kinetics concepts including activation energy definitions, reaction profiles with catalysts, heterogeneous catalysts, and Maxwell-Boltzmann distributions.
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Question text
3 This question is about reaction kinetics.
(a) The best way to describe the activation energy of a reaction is
(1)
A the average energy of the particles when they react
B the difference in energy between the reactants and the products
C the minimum energy required to make the particles collide
D the minimum energy required for a reaction to occur
(b) The diagrams show two reaction profiles for the same reversible reaction
involving gaseous reactants.
Shown on each diagram are the reaction profiles for the pathway without a
catalyst and the pathway catalysed by a heterogeneous catalyst.
(i) In which diagram does the arrow represent the activation energy for the
backward reaction when a catalyst is present?
(1)
A B
500 500
400 400
Enthalpy 300 Enthalpy 300
/ kJ mol−1 reactants / kJ mol−1 reactants
200 200
100 products 100 products
Reaction pathway Reaction pathway
C D
500 500
400 400
Enthalpy 300 Enthalpy 300
/ kJ mol−1 reactants / kJ mol−1 reactants
200 200
100 products 100 products
Reaction pathway Reaction pathway
(ii) Estimate, using the diagram, the decrease in the activation energy for the
forward reaction when a catalyst is added.
(1)
8 450
400 *P62307A0828*
300 reactants
Enthalpy
−1 250
/kJmol
100 products
Reaction pathway
A 75 kJ mol−1
B 100 kJ mol−1
C 175 kJ mol−1
D 200 kJ mol−1
(c) State why a solid (heterogeneous) catalyst is suitable for a reaction in the gas phase.
(1)
(d) The diagram shows a Maxwell‑Boltzmann distribution of molecular energies for
gaseous molecules.
Ea (catalyst)
Ea
Number of molecules
with a given energy
Y 9
Z
*P62307A0928*Energy,E
(i) Which is the area of the graph corresponding to the number of molecules
with sufficient energy to react when a catalyst is present?
(1)
A Y
B Y − Z
C Y + Z
D Z
(ii) Which would always result in a decrease in the number of molecules
contained within area Y?
(1)
A decreasing the temperature of the gas
B increasing the pressure of the gas
C putting the gas in a smaller container
D removing a quarter of the catalyst
(Total for Question 3 = 6 marks)
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How to answer it
Reaction Kinetics Study Guide
What this question tests
This question assesses core understanding of reaction kinetics, specifically definitions of activation energy (Ea), interpretation of enthalpy profile diagrams for forward and backward catalysed/uncatalysed reactions, the role of heterogeneous catalysts, and the interpretation of Maxwell-Boltzmann distribution curves under varying conditions.
Definition of Activation Energy
✅ Correct Answer
D: The minimum energy required for a reaction to occur.
❌ Common Errors
- A: Incorrect because it refers to the average energy of particles, not the minimum threshold.
- B: Describes enthalpy change of reaction (ΔH), not activation energy.
- C: Particles colliding with this energy won't necessarily react unless they meet or exceed the activation threshold.
Reaction Profiles: Backward Catalysed Activation Energy
✅ Correct Answer
C
The arrow starts at the products level and points straight up to the peak of the catalysed pathway curve.
💡 Key Knowledge
Activation energy for the backward reaction always goes from the products up to the peak. Because the question specifies a catalysed reaction, you must measure up to the lower (catalysed) peak.
Calculating Activation Energy Decrease
✅ Correct Answer
B: 100 kJ mol⁻¹
📐 Step-by-Step Calculation
- Find uncatalysed peak energy: Reactants are at 275 kJ mol⁻¹, peak is at 450 kJ mol⁻¹. Uncatalysed Ea = 450 - 275 = 175 kJ mol⁻¹.
- Find catalysed peak energy: Peak is at 350 kJ mol⁻¹. Catalysed Ea = 350 - 275 = 75 kJ mol⁻¹.
- Calculate decrease: 175 - 75 = 100 kJ mol⁻¹ .
Heterogeneous Catalysts in Gas Phase
✅ Correct Answer
Must state that a solid catalyst provides a surface for the reaction.
🧠 Exam Technique
Keep your answer concise. Examiner guidelines explicitly accept "provides a surface". Ignore vague descriptions of adsorption, bond weakening, or alternative routes unless directly tied to providing a surface.
Maxwell-Boltzmann Distribution with Catalyst
✅ Correct Answer
C: Y + Z
❌ Common Errors
Students often select just Y (which represents the extra molecules reacting due to the catalyst lowering Ea). Remember that molecules that could already react (zone Z) still have sufficient energy when the catalyst is present!
Shifting Area Y
✅ Correct Answer
A: Decreasing the temperature of the gas.
💡 Key Knowledge
Area Y represents molecules with energy between the lowered activation energy and the original activation energy. Decreasing temperature shifts the Maxwell-Boltzmann curve to the left, reducing the proportion of molecules exceeding the activation energy threshold.
Topics
Physical Chemistry · Topic 9: Kinetics I
Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.